1. Field of the Invention
The invention relates to the field of aviation bypass turbojets.
It relates more particularly to a bypass turbojet comprising, a primary duct, a secondary duct, a low-pressure compressor, a high-pressure compressor, and a structural intermediate casing arranged axially between said low-pressure compressor and said high-pressure compressor, said intermediate casing being equipped at its periphery with a plurality of support arms having platforms which internally delimit the secondary duct, in which bleed-off means are provided, allowing some of a gaseous stream delivered by the low-pressure compressor to be tapped off and led to the secondary duct.
The intermediate casing also has, in its radially internal part, a plurality of radial arms connecting the annular walls internally and externally delimiting the primary duct. These radial arms are intended to transmit to the support arms, via the radially external part of the intermediate casing, the forces exerted by the fan and the low-pressure compressor on their bearings, these bearings being carried by shrouds secured to the internal annular wall of the intermediate casing.
In addition, the intermediate casing generally bears a device for driving the turbojet equipment.
The intermediate casing has to have great mechanical strength while at the same time remaining light in weight. This intermediate casing is generally produced in a single piece as a casting or as a welded mechanical construction.
In a bypass turbojet with a high bypass ratio, there is provided, in the primary stream from upstream to downstream in the direction in which the gases flow, a low-pressure compressor followed by a high-pressure compressor which conveys compressed air to a combustion chamber in which the air is mixed with pressurized fuel. There, the fuel is burnt to yield, downstream of the chamber, energy for a high-pressure turbine which drives the high-pressure compressor, then for a low-pressure turbine which drives the fan and the low-pressure compressor. The gases leaving the turbines supply residual thrust which is added to the thrust generated by the gases flowing through the secondary duct. It is these thrusts which are needed to propel the aircraft.
Under certain flight conditions, at part load, for example, while the aircraft is descending, the amount of air delivered by the low-pressure compressor may be too high for the engine to operate correctly. Thus it is necessary to divert some of this air to the secondary duct, to avoid the phenomenon known as surge which is due to the detachment of the fluid streams along the vanes, as this leads to instability in the flow.
Furthermore, when the aircraft is passing through high-volume clouds, quantities of water in the form of rain or hail may then enter the compressors. If the engine is at wide-open throttle, the combustion chamber is supplied with a high fuel delivery, and the water is vaporized and is in a sufficiently hot and atomized vapor state not to extinguish the combustion chamber. By contrast, if the aircraft is in the descent or approach phase prior to landing, because the engine is running at idle, the amount of fuel delivered is relatively low, and the compression ratio of the compressors is relatively low. Water in the liquid or solid state may therefore reach the combustion chamber and extinguish combustion of one of the burners, or even all of them. This can have serious consequences.
2. Summary of the Prior Art
This is why bleed-off devices for turbojets are generally equipped with mobile scoops which, under the action of complex control members, can enter the primary duct in the annular space separating the low-pressure compressor from the high-pressure compressor. This annular space axially has the shape of a swan neck, and the particles of water which, because of their specific mass, run along the external wall of the primary duct are trapped by the scoops and diverted to the secondary duct.
GB 2 259 328 discloses such a bleed-off device in which the scoops are actuated by a synchronization device arranged in the intermediate casing so as to direct the tapped air and the particles toward fixed tubes which remove them into the secondary duct downstream of the support arms of the engine.
EP 0 407 297 provides, in the internal and external walls of the inter-duct casing, hatches actuated in synchronism and able to move radially outward.
EP 0 374 004 also provides, in the external wall of the primary duct, hatches associated with a scooping device.
In all these bleed-off devices, the members for controlling the scoops and the hatches are arranged in the inter-duct casing and operate in synchronism. These control members, comprising a control ring, link rods, hydraulic rams and cables to actuate articulated scoops or hatches, are relatively complicated and also difficult to access during inevitable servicing and maintenance operations.
The arrangement of these scoops or hatches and of the control members on the intermediate casing necessitates the intermediate casing comprising means for supporting the articulations of these devices, and this makes the intermediate casing considerably more complicated and considerably more difficult to machine.